 So this next lesson is now focused on the details of beam calibration for high energy photons. As I had seen that a lot of participants are doing this and they certainly know everything about beam calibration. So now I repeat some things. Maybe I can address some things which I have experienced as a personal experience, especially with beam positioning with another position of a chamber. That can be quite tricky and you will exercise today. So in the following dosimetry means, because the dosimetry has so much interpretations, here the dosimetry means the determination of observed dose to water under reference conditions in the clinical beam of the radiation delivery unit accelerator using calibrated ionization chamber. This is what I mean with dosimetry. And this is also frequently referred to as beam calibration. I want to, this is a quantum principle of a calibration procedure, performance of a calibration procedure, correction status and determination of the radiation, quality Q or the quality index which is required for what? There are so many people who know that. Why we need the Q, the quality of the photons? Why we need that? Yes, that can be an answer. You need a quality correction factor, KQ, and the KQ is dependent of Q. So therefore we need this, a measure for radiation quality. Now let me start with some words. We need a protocol to do that. Dosimetry protocols or code of practice state the procedures to be followed when calibrating a clinical photon outcome beam. The joys of which protocol to use can be left to individual radiotherapy departments or jurisdiction of individual countries. I think that in many countries now the code of practice TRS398 developed by the International Atomic Energy Agency is now in use. There are some countries which do not use it. America, they have their own protocol. Germany has their own protocol which is also valid in Austria and Switzerland. Even in the Scandinavian countries for a long time they had their own protocol but now they are switching to the TRS. So there are some debates on certain things and they cannot agree how to manage these different opinions on certain problems. So therefore the Americans have their own protocol and also in Germany we have some things which we think can be done in a better way. Whether this is true, I don't know. Dosimetry protocols are generally issued by national, regional and international organizations. So there are some examples for nationals. The UK has its own, okay. We will wait, okay. We have national protocols, regional protocols. So American Association of Medical Physics is for North America and for Canada. We had for a long time a protocol in Netherlands and Belgium and all the Nordic Association of Clinical Physics and we have this international code of practice issued by the International Atomic Energy Agency called TRS 398. This is an important message. Dosimetry protocol provides three essentials. It provides the formalism. It provides the procedure and it provides what else? The data which you need. So it provides all the data and tables which are required. To use a calibration ionization chamber traceable to standard laboratory photosymmetry. There are, that's not true anymore. 20 years ago we had two different type of protocols. One are using ionization chambers, calibrated in air camera and I think they may be still in use in some countries. Is it true? Are all your chambers now calibrated in water? Yeah. It was not two, ten years ago. It was, I remember also in Bangladesh at the beginning. Yeah, yeah, yeah. Protocols based on the calibration factors in absorptors to water. The way the calibration in air camera is by far the most oldest method. And what is the reason for that? Because it seems to be very practical and natural if we want to measure, absorb those while we should use an ionization chamber calibrated in air camera. That seems to be a detour. The reason is that in the primary standard laboratory the measurement of air camera was established in a very good way and very precise. Establishment of measurements of water absorbed those in the primary standard laboratory for a long time was not doable. Because you cannot use an ionization chamber because it must first calibrated. It's impossible. So you have to introduce a method to measure, absorb those with different devices, different ionization chambers. And one famous thing is water calorimeter, which is now well understandable. It works very fine. The other one was the free air chamber, which works for small energies, but not for high energies. The fire day cap, where you can irradiate with electrons and you can absorb all electrons. That was one. I think it was the two most important things for the establishment. We say it's the establishment of water absorbed those as a primary. So only if it's possible to have a very good system now based on a water calorimeter, now primary standards have the capability to say this is really the water absorbed those. And therefore now we can calibrate them. But there's some history to that. So the principles are now the following. Suppose that the dose in water is well known at 5-centim depth in the water phenomenon under so-called calibration conditions. And these are the both calibration conditions. The beam quality is cobalt-60 radiation. We have a field size of 10 by 10. SSD is 100 centimeter. We have a water phantom measuring depth in water is 5 centimeter. Positioning of a cylindrical chamber with a center and a electrode at measuring depth. Very important. This is calibration condition. Assume we have, we know the dose under these conditions. A cylindrical chamber is then placed with a center at the depth of 5-centim in the water phantom. Then we can simply obtain the calibration factor by the known dose to water by our measurement. What we have is the charge under reference conditions. So we had to refer to certain air pressure, air temperature. The calibration effect in the ionization chamber is almost negligible. Polarity effect is another story which we will cover later on. So if we do that we can simply obtain the calibration factor. And now the dosimetry is extremely simple. If we have an ionization chamber calibrated with this calibration factor which is called NDW and the Q0 refers to cobalt 60. We measure charge, we correct for this air pressure and multiply with air pressure. So it cannot be more simpler. So dosimetry is extremely simple. So this is reading of the dosimetry corrected for our influence quantities. That means air pressure and so on. And this is a calibration factor which is just defined as I've shown one slide before. So this is an example of such a calibration factor if you buy an ionization chamber. This is an example from I think from Carnitronics, Wellhofer or now Iber. And they tell you here this is NW given a value. By the way, by the way you see the uncertainty here is 2.2% quite a lot. And in any protocol you will find this number, you will find the uncertainty. But 2.2% calibration factor, is that really true? No. Again I will refer to this uncertainty concept. There's a document, it's called GUM, the guide for the expression of uncertainty and measurement. You can find it in the internet. And I really command you to try to copy it, to take it out. GUM, G-O-M, guide for the expression of uncertainty and measurement. And there is, many things are to say, but they are saying the uncertainty should be expressed as standard uncertainty. Can be expressed as relative standard uncertainty. Relativity means that you have uncertainty in percent. Standard means it refers to a probability of the standard of the variance. And you know that if you have a variance, 67%, probability you have that the value is within this plus minus, 67%. But you can also express the uncertainty with a so called coverage factor. So that you have a higher probability that the uncertainty is in. If you use, and you look carefully here, it's very difficult to read. So the reported expanded uncertainty, so this is a so-called expanded uncertainty with a coverage factor of 2. Meaning that the standard uncertainty is just 1.1%. And it's expressed as an expanded uncertainty with 2.2%. So don't make a mistake to take this as a standard uncertainty. So this chamber is now to be used in a beam with another quality cue such as high energy photons and high energy electrons. So this is a formula for the definition of absorptors to water in cobalt 60. And for other words, we simply add a correction factor. This is the famous KQ correction factor, which is taken into account that we do not have now the reference in radiation quality, which is cobalt 60. We are now applying it to any other correct radiation type. So this correction factor depends then from the energy, you can also say quality. Quality in our meaning, the energy or the energy distribution, the spectrum. But it all depends from the construction of the ionization chamber. Each chamber has a specific own KQ value and also depending on energy. It's called the beam quality correction factor here. So this is now the principle of the calibration procedure. The dose in water is our measure of charge, the calibration factor and this KQ. So again, the symmetry is very simple if we have knowledge on this. So very frequently the common reference quality is cobalt 60. I think in 99% of KQ values, which are offered, the quality used for calibration is cobalt 60. But the French primary standard laboratory is also offering ionization chambers calibrated in electrons. And there's a tendency now, or even the MPL in the UK is offering ionization chambers calibrated in other qualities. And I think there's a tendency who has a cobalt 60 is decreasing now. Many institutes do not have anymore a cobalt 60 machine. So why we should have a calibration factor based on cobalt 60? So I think the Switzerland, they are trying to introduce ionization chambers which are offering calibration factors directly for 6MV or 15MV. So I think in 10 years we will not have any more KQ values given here, but we will have now more modern KQ values. So for cobalt 60 we say the KQ is automatically left and then it means that the calibration quality was cobalt 60. How to get the beam quality correction factor, this KQ, how to get it? And if you look in the TRS protocol, they think the first choice is a measured one. So some, I told you already, some standard laboratories are able to offer you a measured, but this is only very rare. So the second choice where no experimental data are available or it's difficult to measure KQ directly, calculated correction factors have been used. And the KQ values which are provided in the TRS protocol are all calculated KQ values. So the properties of KQ is the values of KQ are dependent on the quality of radiation. That means if you consider the energy, but other than machine, if you have different machines with same energies, say 15MV variant and 15MV from electric, maybe different in the spectrum, of course. Now each machine is producing different energy spectrum. You always... Yeah, it can be different. So we cannot use a KQ value for 15MV photons. It's impossible because it may be different from... Another thing, we never know exactly 15MV photons how they are produced with the 15MV, MEV electrons. MEV electrons hitting the target, but we never know exactly whether it's 15 or 15.1. We have no idea what is the real energy. We have no idea what is the energy spread. We have no idea on the geometric ring. We don't know nothing about the primary electrons. Therefore the quality, that means the spectrum of photons coming out is always depending on the primary columnator. It's depending on the secondary columnator. It may be dependent on the multi-leaves and so on and so on. So the spectrum coming out is different for each. So therefore, later on, we need something more precise to know what is meant with this Q. And each type of anization chamber needs a particular KQ. Again, it's a little bit strange why this is the case. Because the details of construction of an anization chamber has also an influence of the secondary electrons which are finally counted in the air cavity. If you have a thick wall made of PMMA or you have a graphite wall or whatever, all these details are influencing the finally the measured charge. So each chamber has a different KQ. So therefore we have normally such tables. So we have different anization chambers here. And we have the KQ as a function of the beam quality. And then I have not put the numbers in because I want to tell a little bit more on beam quality. The top-dose order is to be termed in the point P in the order in the reference depth. So if we look in the absence, so okay, this is also some very interesting point. In the absence of the chamber, the dose is given by this expression. The point, the measurement point is at reference depth. That is meant by this expression. Using the chamber, the dose is given by this expression. The question now is, what we are measuring with the chamber is the integral of all this. And how we have to position the chamber now. We can imagine if we put the anization chamber a little bit higher, it will be more dose if you go down. So how to position the chamber? This is a discussion of that. Here we have our anization chamber like that. This is a sensitive volume. Is this correct? Or is this correct? This one? This one is okay? The answer is a little bit strange. If you perform a depth dose measurement, this is correct. The effective point of measurement. But for beam calibration, it does not matter. Because it is taken into account in the KQ value. You can put all these things in the KQ value. So that is just an explanation. If it is filled with air, there is no attenuation. There is a perturbation of the dose fall up in the water, introduced with the air cavity. So we have here our cavity. So this is now a demonstration, a little bit exaggerated, if you have introduced an air cavity. Which positioning is correct? So one may think that the correct way is to position the chamber at its effective point of measurement. You said this already. This is true for depth dose measurements. But for a calibration measurement, it does not matter as long as the positioning is well defined. And any deviation of the correct positioning is taken into account in the calibration factor. Now that is the truth. Calibration factor or in the quality correction factor. And now it is interesting. We do have different protocols and we have different prescriptions now. How to position a chamber, especially a cylindric chamber. And I can already tell this now for photons following the TRS, the prescription. And that is important. It is not a scientific reason for that. Only the prescription is position the center of the cylindric chamber in the depth of measurement. Following TRS for photons. Following TRS for electrons. The prescription is position the effective point in the measurement. In the German protocol, we do have a prescription position always the effective point. Whether it is photons, electrons or cobalt-60. Even for cobalt-60. So again, I think this illustrates the fact that you can do what you want. But you have to follow exactly the prescription. That is an important thing. So if you want to follow the TRS remain, you have to follow the prescription which is written down. And I remember it is just maybe fun, but it is not really fun. Again, I am not telling where it was. It was in the hospital somewhere. People showed me their method of calibration of electrons. And they told me, we don't wish to get always our hand wet with water. We are using a phantom, a solid phantom. We will discuss this problem later on. What is it? Some problems. We do a calibration at the maximum dose. We do this at the maximum dose. Using the KQ which is given in the protocol, which is completely wrong. So again, this is my method. You must exactly follow the prescription which is laid down in the protocol. They say very clearly what has to be done. So this is, and the positioning again, it is very easy to prescribe in terms of the so-called reference point of the chamber. So for cylindrical chambers, the reference point is in the center of the cylinder at the central electrode. This is called the reference point of the chamber. And this point is normally made visible by the company. They include some information, a ring, a black ring or something else, saying this is the reference point of the chamber. And it's important to know because the prescription is, positions the reference point or positions the effective point at the measuring depth. So this is for plane pearl chambers. The reference point is on the center of the front surface at the inner air cavity. Okay, again one remark to that. We all assume that if we do depth dose measurement with a plane pearl chamber, if we adjust our reference point, then we get directly the correct depth dose. This is our assumption that it was written everywhere. Now we know more and more that using the markers chamber, where is the inner surface if you do it in water? It's not so clear. If you use rose chamber, again it's not so clear. The effective point is in reality it's a little bit lower than its inner surface. 2.2 millimeter. But you can see it if you compare, say, Monte Carlo depth dose curves, which you think are true, and you compare it with a plane pearl chamber, with a rose chamber, which is positioned like that. You see a small, you can directly see the difference. So there's a small shift. So positioning can be done. Now we define as an adjustment of the reference point of the chamber with respect to the measuring depth. And we have these prescriptions for the purpose, for the beam calibration. We have for cobalt 60 at measuring depth. So the reference, this is the central axis, should be at measuring depth for high energy photons at measuring depth and for high energy electrons. It must be 0.5 times the radius deeper than the measuring depth, the so-called effective point of measurement. For depth dose measurement, it should be 0.6 times the radius deeper than the measuring depth for cobalt 60, for high energy photons, 0.6 deeper and 0.5 deeper. So it is a little bit mixed, strange. But this is prescription. You have to follow it. This is then for plane pearl chambers. And it said for beam calibration and for depth dose measurement, the position always the reference point, that is not clear, the reference point should be at the measuring depth. So it's shown here. This is a depth of measurement of interest. This is a position for cobalt 60 and high energy photons. Okay, done. But positioning seems to be such a stupid thing, but you can make so much mistakes with that. So let's do a little bit of exercise later on. So performance of a calibration procedure. The procedure now appears quite simple. Take an ionization chamber for which a calibration factor is found and certificate. Adjust the gentleman in the water following the precision protocol, obtain charge under reference conditions. That means the charge has to be corrected for air density, air temperature. You have to take into account the saturation polarity effect, obtain KQ from an appropriate lookup table, which is offered in the protocol, and then it's done. Again, quite simple. There is only two points left. What exactly means obtain charge under reference conditions? I have already told that. The second one is we have a lookup table, but how we get a quantity value for the quality KQ. So we need a procedure to determine a quantitative measure for the beam quality. This is important. The KQ values, which are offered in protocol if you want to use, are only valid under reference conditions. If you deviate from them, one example is this measuring for electrons at maximum depth, which is not correct. There is prescription. We came to this later on. If we go to the calibration electrons, you have to do the reference depth for electrons at a certain depth, and only there the KQ is valid. If you go to the other one, it's not valid. The same is true for photons. We have reference depth for performed measurements of how deep 10 cm are, in most cases. In Germany, we have only 10 cm. In the T-Rest protocol, you can only use for a low energy of 5 cm. Why in such a deep, why so deep? Why not in those maximum? Why we do measurements in 10 cm depth? This is an argument using the prescription. We have to follow the prescription, but there is also a scientific reason for that. That is the case, the electron contamination. This can be quite different. We have a lot of electrons coming down, or say we have a different number of electrons, which is again not clear. It may depend again from the construction of the gantry. We get rid of all the electrons in 10 cm depth. Therefore, the prescription is a 10 cm depth. This is now taken directly from the document. It's saying the reference condition for photons, phantom material should be water, phantom size, this size, source, chamber distance, 100 cm. Alternatively, I think this is not well done. I think it should only be ssd 100 cm. This is saying the distance between the focus and the chamber is 100 cm. I'm not happy with it. In Germany, we have only one ssd, and that's it. That is a little bit of a small ambiguity. Air temperature at 20 degrees, air pressure at P01, P01.3 kPa, reference point of the chamber. For cylindrical chambers, the chamber axis. For plane-bore chambers, it's in the surface of the entrance windows. Depths and fandom of the reference point of the chamber. What is this? I don't know. Field size at the position of the reference point of the chamber. Again, some other documents are saying that it's a field size at the surface. Polarizing voltage as a calibration certificate, those rate, there are no reference values. So we have, of all these influence factors, we have mainly these three, the air temperature, the air pressure and the polarizing voltage, polarizing voltage polarity. In order, we cannot do measurements under these conditions, normally. We have to perform our measurements under arbitrary temperature and pressure. Then we have to introduce these correction factors, which are shown here, assuming that all these different influence factors act independently from each other. A product of correction factor can be applied, and this is here the product of the different correction factors. And I will shortly discuss this most important. This correction factor is one for air temperature and pressure, and you are very well aware of this formula which has to be applied. Okay. How to get the air pressure? You need an instrument there, and it must be calibrated to C level, which is not automatically the case. So you can make, again, mistakes just... You need a careful calibrated instrument for that. For temperature, it's less important. But you can see any mistakes, if you have 1% uncertainty in the measurement of air pressure, it directly goes to the dose determination. If you make an uncertainty of the temperature, it goes only with one-third. So it's important to have a measurement device for the pressure which really is measuring the C level. And there's one trick saying, very often there's an airport in the airport. You can ask them. You can ask them. You can phone. Can you tell me today the pressure? And please give me the pressure. Not on the C level, but give me the air pressure on that ground where I am. Believe me, it would be in 3,000 m height. Or as we have to take the air pressure, not referring to C level, but it must be referred to that level where you are really doing the measurements. And sometimes, again, I told you, if there is an airport in the neighborhood, one can ask them. They normally have good information on that. That may be. You have to check it. That is something which, if you don't know exactly, then you just take it. There's a risk to make a huge mistake. Polarity effect. The formula I think you are well familiar with this formula. However, there is one thing, again, which can go wrong. Has the calibration laboratory really corrected for the polarity effect? If you have your certificate, so you get your calibration factor, has it been corrected or not? You have to look at the certificate. Here's one example. And you look carefully, then you will find they have made no correction. So the consequence is, if it's not done here, the calibration factor refers to a certain polarity. It has not been corrected. So then if you are, if no polarity correction is performed during calibration, then it is included in the calibration factor. That means if the user beam quality is the same as in the calibration quality, so if you do a measurement in common 60, you must not correct the polarity correction. Because then you overdo it. It's not in. So the user must not apply a polarity correction factor for that particular beam. But what is the thing that if it's another, if it's high-energy photons, what then? The polarity influence can go up in electron beams for the Marcus chamber at 10 MeV almost to 1%. So it is something which has to be corrected. So how to get now the polarity correction factor? You have to reproduce the quality of cobalt 60 and do your own measurements. And if you have no cobalt 60, then just use 6 MeV for that. So estimate the polarity correction for the machine, for the quality which is used for the calibration. Ideally it should be cobalt 60. If it's not available, you can use 6 MeV. And the same way you do the same for the user quality, the 15 MeV, and you have to take the ratio of these tools now for the polarity correction. This is the correct way to perform the polarity correction factor. And if you look in the document, it's well written, but it's written some pages after the first equations. And if you are in a hurry, you will look how I have to do saturation correction, I have to do polarity correction, I go through, there's a formula, oh fine, I do the formula just, but you have to follow the readings and see what's going on. Then the next factor, there's always difference in the charge produced by the radiation and the actually measured one. This is due to that there's a recombination of charges. We know that there's positive ions and negative ions and they can hit and then they can recombine. So the number of charge which is really measured is in a rule lower than that which has been created. On the other hand, we are using this W value that the energy absorbed is the number is referred to the created charges. So there are a set of different effects which are saying the recombination of ions formed by separate ionization particles tracks termed general or volume recombination and there's only the initial recombination. There's a theory behind that which is time consuming and it's not so easy to understand. It needs to know that the charge which is measured is less than the charge which is produced and it's all different in pulse radiation like in an accelerator and the code of practice is recommending that the correction factor is done in this way by the so-called two-voltage method. You do a method by the normal voltage say for 400 voltage then you do a measurement with a reduced voltage say 100 and you have different charges. They are clearly different. It's remarkable how you can see that you really see a changed charge just by changing the current. This is the ratio of the measured charges and if you have obtained these ratios you have to apply this formula where here you have different coefficients A0, A1, A2 according to what ratio of charges you have of voltage you have used so if you had in the pharma chamber 400 and 100 volts so the ratio is 4 you have to use these factors to calculate the saturation and please if you do a careful calibration you always have to apply this correction. I have found our colleagues or the Germany are saying ah it's a small effect, we don't need it but a careful calibration definitely requires the correction of the saturation effect. Typically for 6 and V, for a pharma chamber it's 0.3%. It's a small amount but it's something which you really can correct very clearly. This refers now to Koubal 60 which is less important and I will go into a summary if the chamber is used under a condition that differs from the reference condition then the measured charge must be corrected for the influence quantities by so-called influence correction factors and the most important is the KTP and the higher density for the polarity effect and for the saturation effect. So we may do such calibrations in different qualities so the TRS is referring to low energy x-rays to medium energy x-rays these are all qualities which are addressed in the protocol so there are prescriptions and the parameters are available for all these types. Within each category of radiation a particular quantity parameter so-called the quality parameter is defined that may be different the quality parameter KQ is defined in different ways for different types of radiation. Yes. No, it's a clear prescription how to determine the quality again it's a clear prescription we have to follow it it's true it's 10 by 10 for photons for photons so the method to determine the quality parameter differs from one radiation to the other now we come to the photons this is the definition of the quality parameter for high energy photons produced by clinical accelerators the beam quality is specified by the so-called TSU phantom ratio TPR in 20 centimeter depth and 10 centimeter depth and here is again the condition how this has been to be measured so and this shows how this can be done you have here our water phantom you have a 10 centimeter field in the depth of the ionization chamber this is at 100 centimeter depth from here from the focus to the chamber and we have here a 10 centimeter depth then we do a measurement and then we simply fill up the water to 20 centimeters we have two different measurements and TPR is simply which is defined as a dose in 20 centimeter and 10 centimeter in a very good approximation the ratio of the two measurements but there is another method which is using a PDD which is called the PDD method and it is showing here we have our water phantom now a distance from the focus to the surface of 100 centimeter and we have the 100 centimeter field at the surface and we have our chamber at 10 centimeters and we get one result we have it in 20 centimeters and we get one result and we call this the ratio, PDD ratio 20 centimeter, 10 centimeter it's given by this ratio and the TPR is then obtained by this equation by the way the German protocol is saying only use this method the TR protocol is saying use both methods later on in our exercise I recommend to use the second method so and if we have, now we have really a number and by using these numbers we can hear this is the numbers of the beam quality and now we can find all the quality correction factors which are now needed and this is summary of the beam, of the photon calibration this is the equation of how the calibration must be performed in Germany we are always saying this is a matter of all the symmetry equation we call it the matter of all equations follows the positioning instruction of the protocol for depth dose measurement position the effective point of measurement of the chamber at the measuring depth for photons and saying that the effective point of measurement is 0.6 times in Germany we are saying 0.5 times the radius and in the real world you must say no one knows exactly where is the effective point so it is an approximation for beam calibrations positions the reference point of the chamber at the measuring this is for cylindrical chamber and for plane probe chambers these are the most important correction factors the quality correction factor is given in tables provided in the DRS document this is for photons and for high energy photons produced by clinical accelerators the beam quality Q is all the time sometimes called the quality index is specified by the tissue phantom ratio TPF produced photos this parameter can be measured directly or determined by depth dose measurement now I am ready with calibration of photons and again I will make a break for the next talk to the calibration of electrons